The present invention relates to anti-fouling marine coatings and more particularly to novel water-soluble leaching agents therefor.
One of the earliest needs for performance-oriented coatings was in the marine environment. Early formulations were designed around known toxins, such as copper and mercury compounds. 19th Century marine coatings typically used creosote and natural drying oil formulations bearing the toxins. For ship bottoms, presently anti-fouling compounds based on copper and tin commonly are incorporated into somewhat water-sensitive binders to afford gradual breakdown of the film to permit a sustained release of the "poison". This required self-erosion property necessitates frequent repainting of ship bottoms, depending upon location and severity of exposure conditions. With respect to the toxins presently used, toxicity requirements typically restrict anti-fouling agents to be based upon copper and tin, and often combinations of these agents are utilized.
Today's anti-fouling coatings use two general leaching mechanisms, depending on the type of resin matrix selected, soluble or insoluble. The insolube-matrix type leaves a resinous skeleton intact as the toxicant particles are removed by dissolving into solution in seawater. This is also called the contact type because it depends upon the toxicant migrating to the surface and entering solution by making contact with seawater. Since the resins are somewhat water-permeable, the toxic particles may diffuse through the semipermeable coating, and as one particle dissolves, another is exposed to seawater. The contact type contains several times more toxicant than the soluble type. The resultant thicker films of toxicant provide a longer service life to the anit-fouling topcoats. As a general rule, the insoluble-matrix type of paint does not contain an extender pigment, and the geometry of the dry film requires high-toxicant loadings (52 to 74 percent by volume) to ensure that the Cu.sub.2 O partcles will be in continuous contact with each other. Below the level of cubic packing (52 percent), the resin will encase the Cu.sub.2 O particles and prevent solution; above the level of hexagonal packing (74 percent), the coating will be too resin-poor to maintain film integrity. These figures may vary somewhat in actual practice, and it is common to adjust the leaching rate and the effective range of toxicant loading, (e.g., by the addition of rosin or other natural resins). In commercial practice, both natural resins and extender pigments are frequently used. When high levels of rosin are used and high erosion might be expected, tougheners such as ester gum, ethyl cellulose, and modified rubbers are added. "Paint Handbook" G. E. Weismantel, McGraw-Hill, New York, N.Y., pp 14-43 and 14-44 (Chapter 14 "Marine Paints" authored by R. J. Dick).
One problem plaguing anti-fouling marine coatings containing multiple toxicants is the apparent loss of anti-fouling activity expressed by coatings over extended periods of storage. Thus, the need for improved storage stability of anti-fouling marine coatings is manifest. The present invention is directed towards the improvement of the can or storage stability of anti-fouling marine coatings containing ore than one toxicant.